Since vinyl chloride resin has excellent moldability, high mechanical strength and good chemical resistance and is inexpensive, it is broadly used in various fields, including semiconductor production apparatuses, as industrial materials, particularly as corrosion resistant industrial materials.
However, though vinyl chloride resin has fire-retardant property due to its high chlorine content, it has poor heat resistance so that it causes thermal decomposition when exposed to a temperature of 200.degree. C. or more, thereby inducing smoke by organic matter and also generating corrosive gases such as chlorine gas and hydrogen chloride gas. Accordingly, when a fire starts in an apparatus which uses moldings of such a vinyl chloride resin, smoke and gas are generated due to decomposition of said resin.
With the aim of preventing such accidents, a phosphorus based fire retardant, a halogen based retardant or a water-containing compound such as of magnesium hydroxide has been added to vinyl chloride resin to improve its fire-retardant property. However, even in the case of moldings of vinyl chloride resin whose fire-retardant property is improved in this manner, when they are used for example in semiconductor production apparatuses, smoke and corrosive gas generated by a fire reduce the purity of air in the production facility and thereby cause a possible danger of polluting and spoiling production apparatuses, instruments and semiconductor parts.
In view of such situations, fire-retardant property and the ability to inhibit generation of smoke and corrosive gas at the time of fire have recently been demanded even for the vinyl chloride resin said to have fire-retardant property in general, and since such a demand is particularly strong in the field of insurance organizations, the evaluation criteria provided by Factory Mutual Research Corporation that constitutes Factory Mutual System, an industrial mutual insurance organization having a base of operations in North America, are effectively used.
According to this evaluation criteria, a fire propagation index FPI of 6 or less which shows fire-retardant property, a smoke damage index SDI of 0.4 or less which shows smoking ability and a corrosion damage index CDI of 1.1 or less which shows corrosive gas generation are required at the same time, based on a fire-retardant property test of clean room materials (FMRC Clean Room Materials Flammability Test, to be referred to as "FM standard" hereinafter) cited as Class Number 4910.
Regarding these requirements, a fire propagation index FPI of 6 or less of vinyl chloride resin moldings can be achieved relatively easily by the addition of a fire retardant additive, but it is not easy to obtain a vinyl chloride resin molding which also satisfies the smoke damage index SDI and corrosion damage index CDI at the same time, and only extremely few moldings such as the fire-retardant vinyl chloride resin moldings disclosed by the present inventors in Japanese Patent Application No. 9-365319 can satisfy the FPI, SDI and CDI standards at the same time. Such moldings, however, have disadvantages of being not so good in strength, chemical resistance and corrosion resistance.